Various techniques are known which can provide valuable information as to the performance of gas sensors and potentially facilitate the prediction of impending failure or remaining life. Simple stimulus checks, such as electronic pulsing to confirm the presence of a device or ‘bump’ testing with gas can be accommodated within instrument hardware and normal operating regimes. However, more sophisticated hardware and software is required to perform other useful tests, especially those requiring analysis of the response to complex electrical waveforms.
There are a number of non-destructive test methods which may be used to provide enhanced information about the status of sensors and their host instruments, for example:                complex AC impedance analysis of electrochemical cells;        sensitivity/temperature profiles and heater resistance measurements of various heated devices (including catalytic and heated metal oxide sensors);        received signal level profiles to analyze the performance of optical sources and detectors (as used for example in small infra red sensors);        AC analysis of electrical circuits to determine drift in key components (for example capacitance changes in high impedance amplifiers); and        AC impedance analysis to determine the status and performance of the battery within the instrument.        
Many of the above require hardware and software capabilities which exceed those which can economically be provided in small portable instruments. In many cases the respective sensor is unable to perform normal monitoring functions during the test. This may be due, for example, to a requirement to:                provide a known gas environment or other ambient condition/external stimulus;        disconnect the sensor from the usual measurement or display circuitry;        use relatively high power consumption methods;        allow significant recovery time before the sensor can operate normally;        measure external parameters such as gas flow or pressure;        use processor speeds or memory sizes not available in the instruments;        provide hardware/software having no useful function in ‘normal’ measurement operation; and        use methods which would be difficult or complex to include in a certified device.        
Additionally, the interpretation of some test data can be rendered problematic by large temperature excursions which might be encountered in the field. Performing such tests under relatively controlled conditions is therefore greatly preferred. These and other attributes render such functions inappropriate for inclusion in low cost instrument designs or for routine use during normal instrument operation.